Image sensor is a kind of device which can transform optical information into electrical signals. Currently, image sensors are widely used in camera shooting, image capturing, scanning, industrial measure, and the like. Existing image sensors may include charge-coupled-device (CCD) image sensors and complementary-metal-oxide-semiconductor (CMOS) image sensors.
A CCD image sensor may include a stack of a micro lens layer, a color filter layer and, a pixel unit array layer, e.g., photosensitive diodes. Incident lights may focus on the color filter layer through the micro lens layer, then reach the pixel units from the color filter. The pixel units may transform optical signals obtained from the incident lights into electrical signals, transfer the electrical signals through grids among the pixel units, and then output the electrical signals.
Compared with CCD image sensors, CMOS image sensors can be driven more conveniently and can implement various scanning modes. Furthermore, minimization of CMOS image sensors can be realized by integrating signal processing circuits into single chips. Besides, CMOS image sensors can be formed using widely compatible CMOS techniques, which may reduce the power consumption and manufacturing costs. Therefore, CMOS image sensors are used more widely.
Current CMOS image sensors may be classified into two kinds: front-side-illumination (FSI) image sensors and back-side-illumination (BSI) image sensors.
A FSI image sensor may be successively constituted by a stack of a micro lens layer, a color filter layer, a dielectric layer having an interconnection layer formed therein, and a layer including a pixel unit array and output circuit structures disposed in a same layer with the pixel unit array and electrically coupled to the pixel units. Lights incident to the FSI image sensor may be focused by the micro lens layer, having a anti-reflection coating formed thereon, onto the color filter layer and then reach the pixel units through the dielectric layer. The pixel units obtain optical signals, transform the optical signals into electrical signals, and output the electrical signals through a corresponding output circuit. In the FSI image sensor, an output circuit corresponding to each pixel unit may include two metal-oxide-semiconductor (MOS) units. As the lights reach the interconnection layer before the pixel units, and may be diffracted, absorbed and/or sheltered by metal and interdielectric layers in the interconnection layer, light absorption rate of the pixel units will be inevitably reduced. When the pixel units shrink to 1.1 micron scale or smaller, the reduction may severely affect the device's operation.
To address drawbacks of FSI image sensors, BSI image sensors are developed. A BSI image sensor may be constituted by a successively stack of a dielectric layer with an interconnection layer formed therein, a layer including a pixel unit array (i.e., pixel units) and output circuit structures electrically connected with the pixel units, a color filter layer and a micro lens layer. Lights illuminate from the backside of the BSI image sensor, then are focused onto the color filter layer by the micro lens layer having an anti-reflection coating formed thereon, and then reach the pixel units through the color filter layer. The pixel units may transform optical signals obtained from the incident lights into electrical signals, and corresponding output circuits may output the electrical signals. In a BSI image sensor, an output circuit corresponding to each pixel unit may include two MOS units.
However, those existing image sensors, including the CCD sensors, the FSI sensors and the BSI sensors, have drawbacks respectively.
Firstly, grids are needed to implement signal transmission among the pixel units in a CCD image sensor. The CCD image sensor may thereby have a reduced fill factor (a ratio of a photosensitive area to the whole device area) due to the grids. Besides, the grids may shelter the incident lights, resulting in a reduction of light usage. Therefore, the CCD image sensor may have relatively low sensitivity to incident lights. In a FSI or BSI image sensor, photosensitive pixel units and the MOS units of the output circuits are formed in a same layer, which may also limit the fill factor. Further, lights may be sheltered by an interconnection layer in the FSI, resulting in a reduction of light usage. Therefore, the FSI and BSI image sensors may also have relatively low sensitivity to incident lights.
Secondly, it is difficult to optimize the processing conditions of the pixel units in the CCD image sensor, due to the grids processes. For the FSI or the BSI image sensor, since their pixel units and MOS units are formed in a same process, it is also difficult to optimize the processing conditions of the pixel units, due to the limitation of thermal processes for forming the MOS units.
Thirdly, leakage current may occur between the grids and the pixel units in the CCD image sensor, which may also occur in conducting channels beneath the grids. In the FSI or BSI image sensor, leakage current may exist between the pixel units and the MOS transistor. Besides, there may be leakage current existing between the pixel units and the substrate, because normally isolating structures disposed between the pixel units may not be as deep as the pixel units.
Further, the CCD, FSI and BSI image sensors have relatively low resolution attributed to their manufacturing processes, e.g., low system integration degrees and relatively large packaging sizes due to necessity of being packaged with additional chips.
To avoid the drawbacks described above, for example, limitation in further scaling down due to relatively low fill factor, undesired performance due to multi-chip system packaging, high costs, low reliability and the like, three-dimensional contact image sensor (3D CIS) technology are rapidly developed in the field. The technology uses conventional CMOS manufacturing techniques or what are completely compatible with the CMOS manufacturing techniques. In the technology, a plurality of chips, having various functions or same functions, are stacked for bonding along a direction vertical to surface. However, 3D CIS technology still has drawbacks such as low yield and high cost. Therefore, there is need for a 3D CIS solution completely compatible with conventional CMOS manufacturing processes with simple manufacturing processes.